photos

If you are going to let anyone else see the work,the answer is "yes", you generally need permission. However, if you alter it with or without permission, or include it or a version of it in your own work, your image is a "derived" image and you cannot claim sole copyright of the image you make: you have to acknowledge the source of the original if you submit it to a publisher, contest of photo-sharing site, and you need the other artists permission to use their modified work. Check with an attorney!